Otitis externa is a condition characterized by inflammation of the external ear canal. The external ear canal is distal to the tympanic membrane. The ear pinna may or may not be involved in otitis externa.
Ear infections such as otitis externa represent one of the main reasons that dogs are presented to veterinarians, and may affect up to 20 percent of them. Dogs with otitis externa often present with swelling and erythema of the epithelial tissue of the ear canal, increased discharge from the ceruminous glands in the ear, and behavior suggesting pain and pruritus. Clinical signs can include any combination of headshaking, odor, pain on manipulation of the ear, exudate, and erythema. Otitis externa can occur in various mammals besides dogs, including cats and rabbits among others. Environmental factors such as high temperature and humidity can influence the incidence of otitis externa in dogs.
Some dog breeds, particularly those with large or hairy ears like cocker spaniels, miniature poodles or Old English sheepdogs, are more prone to ear infections, but they may occur in any breed. Many dogs will have more than one type of ear infection present (e.g., a bacterium and a fungus, or two kinds of bacteria). Many dogs with chronic or recurrent ear infections have allergies or low thyroid function (hypothyroidism).
The external ear canals of most dogs and cats harbor small numbers of commensal gram-positive cocci. Gram-negative bacteria are also present. These organisms may become pathogenic if the microenvironment is changed and encourages their overgrowth. Aside from bacteria, other primary causes of otitis externa include allergy, autoimmune (e.g., pemphigus), endocrine dysfunction, epithelialization disorders, foreign bodies, glandular disorders, immune-mediated (e.g., drug reactions), fungal (e.g., aspergillosis), parasites, viral (e.g., canine distemper), and miscellaneous (auricular chondritis, eosinophilic diseases, juvenile cellulitis, proliferating necrotizing otitis of cats).
Infectious bacteria in otitis externa can include Staphylococcus aureus or Pseudomonas aeruginosa but in many cases there are numerous types of bacteria observed. Coccal organisms are usually staphylococci or streptococci. Rod-shaped organisms are usually Pseudomonas aeruginosa, Escherichia coli, Proteus vulgaris, or Proteus mirabilis. The presence of many neutrophils confirms the pathogenic nature of the organisms. Methicillin-resistant Staphylococcus intermedius and Pseudomonas otitis (caused by Pseudomonas aeruginosa) have emerged as frustrating and difficult perpetuating causes of otitis because of the development of resistance to most common antibiotics. These infections are often chronic in course (>2 month) and associated with marked suppurative exudation, severe epithelial ulceration, pain, and edema of the canal.
Infectious yeasts that cause otitis externa include Malassezia pachydermatis, Aspergillus niger, and Candida albicans, among others. M. pachydermatis is found in low numbers in the ear canals of many healthy dogs and cats. Because yeasts colonize the surface of the ear canal, they are most easily found adhered to clumps of exfoliated squamous epithelial cells. M. pachydermatis is identified readily on microscopic examination and its numbers easily assessed. There is no specific number that indicates yeast overgrowth. The key determining factor is whether the ears are pruritic.
Otitis externa can also be caused by viral infections that can be from herpes simplex and herpes zoster infections, among others.
Agents, compositions, and methods of treating otitis externa are needed especially caused by antibiotic resistant strains bacteria.